CoStar Group has a well-earned reputation as one of the most litigious companies in real estate. The company’s fight against Xceligent in the mid-2010s became the defining example of its willingness to use the courts to protect its digital assets. In that case, CoStar accused Xceligent of systematically copying tens of thousands of its listing images and republishing them on its own platform, with evidence showing Xceligent had hired a firm in the Philippines that was directed to bypass CoStar’s security measures. A federal judge ruled against Xceligent for widespread infringement, and burdened by the judgment and mounting debt, Xceligent folded in 2017. More recently, CoStar sued Zillow itself for copyright infringement. In July 2025, CoStar filed a lawsuit against Zillow, claiming that Zillow stole tens of thousands of copyright-protected pictures for use across different platforms, with CEO Andrew Florance stating “They wanted to get into the multifamily business and they did it by using our assets and taking shortcuts.”

This history of aggressive litigation might suggest CoStar is simply another competitor using courts to attack rivals. But CoStar’s recent amicus brief in the current Zillow antitrust case against MRED and Compass reveals something more fundamental. The company isn’t defending competitive turf. It’s defending the foundational MLS system that all portals depend on.

The distinction between CoStar’s past litigation and its current position is crucial. When CoStar sued Xceligent and Zillow, it was protecting specific intellectual property that it had invested heavily in creating—photographs shot by professional photographers with CoStar’s money, carrying CoStar’s watermarks. Those were discrete assets that competitors had misappropriated. The lawsuits enforced property rights that everyone agrees should exist. Intellectual property protection, while sometimes aggressive, operates within an accepted legal framework.

What CoStar is now opposing with Zillow is something categorically different. Zillow’s antitrust case against MRED and Compass, if successful, would fundamentally reshape how the entire MLS system functions. Zillow filed a federal antitrust lawsuit against MRED and Compass, alleging the Chicagoland MLS and the nation’s largest brokerage conspired to withhold listing data and pressure Zillow to carry private “hidden” listings nationwide. The underlying conflict traces back to April 2025 when Zillow created listing standards prohibiting agents from marketing listings outside its platform, specifically targeting Compass’s three-phase marketing strategy and MRED’s private listing network.

CoStar’s amicus brief argues that Zillow wants exclusive benefits on both sides of the market simultaneously. Zillow demands unrestricted access to all MLS listings from every region while hoarding its own pre-market inventory through Zillow Preview, a product that lets agents publicly market listings before formal MLS submission. Gene Boxer, CoStar’s general counsel, articulated the core complaint. “Zillow wants a court order forcing MLSs to hand over their listings while Zillow hoards its own exclusive pre-market inventory—a breathtaking ‘heads I win, tails you lose’ proposition,” Boxer stated in the brief. “The Court should see this motion for what it is: an attempt to weaponize the judicial system to entrench Zillow’s dominance at the expense of competition, consumers, and the MLS system that has served the industry for decades.” Despite the obvious hyprogracy of CoStar accusing another proptech giant of using the legal system to damage its competitors, this argument doesn’t lose credibility because CoStar has sued competitors before.

CoStar also challenges Zillow’s claims about consumer protection and fair competition. According to CoStar’s brief, Zillow “dupes consumers by presenting a ‘Contact Agent’ and ‘Request a Tour’ button beside property listings, and the vast majority of consumers do not understand this deception, evidenced by a recent study that found 99.7% of respondents incorrectly identified whom they were contacting on the Zillow platform.” CoStar argues that Zillow Preview is hypocritical, claiming Zillow “audaciously complains about brokerages ‘walling off the listings in their large networks from outside competitors’ and ‘using their large networks to lure buyers and sellers, capturing so-called network effects,’ but the practices that Zillow vociferously condemns describe precisely Zillow’s own behavior and objectives with respect to Zillow Preview.”

Boxer expanded further. “This is not a company defending consumer choice,” he said, “this is a company that the FTC already sued for paying a rival to stop competing, that faces class actions for steering consumers to overpriced mortgages, and that feasts on the 99.7% of its users who don’t know who they’re contacting when they click ‘Contact Agent’ on Zillow’s platform.”

The amicus brief exposes a distinction between competitive tactics that harm individual rivals and those that threaten shared infrastructure. Zillow captures roughly 75% of residential real estate traffic and generates revenue from both listing syndication and buyer-side lead generation. That dominance allows Zillow to invest in exclusive products and partnerships while demanding universal access to competitor listings. If Zillow succeeds in its lawsuit, it essentially tells every MLS in the country that dominant platforms can refuse to share data from competitors while demanding access to everyone else’s inventory. The MLS system was built on the principle that all brokers contribute listings for mutual benefit. Zillow’s position—force all MLSs to provide universal access while maintaining proprietary control over pre-market listings—inverts that principle entirely. CoStar’s litigation history against companies stealing its photographs is orthogonal to defending a shared system against fragmentation by a dominant player.

What distinguishes CoStar’s current position is the stakes involved in the decision. When CoStar sued Xceligent or Zillow for copyright infringement, those cases affected specific business relationships and competitive positioning. The MLS system, by contrast, is foundational infrastructure that thousands of brokerages, agents, and portals depend on. A ruling that allows Zillow to demand access while restricting others fundamentally breaks the system for everyone except Zillow. A court that rules for Zillow will be another invitation to dominant proptech platforms that they can use litigation and market power to reshape the tech landscape in their favor, a slippery slope that we might already be sliding down.